Code division multiple access (CDMA) is a form of spread-spectrum technology that was initially used by the miliary to perform anti-jamming, anti-intercept, and other communications applications. Over the last several years, this technology has been the focus of extensive research, mainly because of its suitability to personal wireless mobile communications. CDMA-based systems, for example, provide significantly higher bandwidth efficiency for a given spectrum allocation compared with conventional systems. They also demonstrate more favorable power consumption and control properties. This, in turn, ensures that a high-level of transmission quality can be sustained over longer periods of time. CDMA systems are also free from geographical constraints, i.e., many conventional systems must use different frequency allocations based on, for example, cell location. Because a coding scheme is used across a broad spectral range, CDMA systems are free from these restrictions.
Operationally speaking, unlike their conventional predecessors CDMA systems communicate signals over a broad frequency spectrum. Prior to transmission the signal is encoded with a set of digitally generated pseudo-noise (PN) codes. This causes the signal to be effectively scrambled across the spectrum in a virtually undetectable manner. At the receiver side, the signal is recovered through a suitable demodulation and despreading scheme that is based on the same set of PN codes. By using these codes, the receiver is able to eliminate noise in a way that allows the voice and/or data content of the signal to be recovered.
The latest advances in spread-spectrum communications are being made by the so-called 3rd Generation Partnership Project (3 GPP). This initiative is made up of a consortium of companies from around the world which have gathered to develop a standard protocol for wireless code division multiple access (W-CDMA) communications.
In order for a W-CDMA receiver to receive a spread-spectrum signal, it must first synchronize the timing of the signal with the transmitting station. In accordance with the 3 GPP standard, initial signal acquisition of the receiver is established through a synchronization channel (SCH), used for cell search and timing acquisition, and a common pilot channel (CPICH), used for determining a scrambling code of the transmitting station and a phase reference of the signal. The SCH consists of two sub-channels, a primary synchronization-channel (P-SCH) and a secondary synchronization-channel (S-SCH). The primary sub-channel typically transmits a 256-chip modulated code to establish a primary synchronization code (PSC). The secondary synchronization-channel transmits a 256-chip modulated code to establish a secondary synchronization code (SSC).
In order to retrieve information from the transmitted signal, the receiving terminal must first detect the primary synchronization code, so that the timing of the receiving terminal may be synchronized to the 667 microsceond burst timing of the transmitter. Once burst timing synchronization is complete, the receiver detects the secondary synchronization code. This code determines one of 64 PN-code groups to which the scrambling code belongs which is used to modulate the transmitted signal, and it also determines 10 msec. frame timing for the W-CDMA signal. After the code-group is determined from the secondary synchronization channel, the final scrambling code out of 8 per PN-code group is determined by correlating the signal with all 8 possible scrambling codes belonging to the code group. This code may also be used to demodulate a downlink signal transmitted on the common pilot channel to thereby identify the transmitting station.
One key problem associated with the initial acquisition of a W-CDMA signal relates to the frequency accuracy of the receiving terminal. A typical frequency reference for a cellular handset has an accuracy between 2 and 5 parts per million (PPM). This results in a carrier frequency error at 1900 MHz operating frequencies of up to about 10 KHz. This accuracy is readily obtained in very small crystal oscillators costing several dollars and is suitable for use in cellular handsets. Base stations, on the other hand, require much greater accuracy typically on the order of 0.05 PPM or only about 100 Hz of error at 1900 MHz. Using equipment that can attain this accuracy has significantly increased the cost of W-CDMA systems. It is therefore desirable to relax the frequency accuracy requirement for initial acquisition in order to lower the cost of the receiving terminal and improve robustness.
Various methods have been developed for improving accuracy in a spread-spectrum system. U.S. Pat. No. 5,950,131 to Vilmur, for example, discloses a method for performing fast-pilot channel acquisition using a matched filter in a CDMA radiotelephone. According to this method, a matched filter is split into multiple matched filters in a linear arrangement with short chip-match lengths. This improves frequency robustness for mapping the PN demodulation code to the modulated signal. The Vilmur approach, however, has proven to have significant drawbacks. For example, this approach is unable to distinguish between codeword ambiguities, depending upon codeword design of a specific system. The inability to resolve these ambiguities substantially affects the performance of the matched filter and therefore the overall system. (The Vilmur method is discussed in greater detail below, and specifically corresponds to a conventional system which is unable to distinguish between sequences 0 and 8 shown in FIG. 6 during initial signal acquisition.)
In view of the foregoing considerations, it is clear that there is a need for an improved method for performing initial signal acquisition in a spread-spectrum communications system, and more particularly one which minimizes receiver errors attributable to frequency offset.